Download Changes in Chromosome Structure

Changes in Chromosome
Structure
Chromosome Structure
Mutations
Mutation
Description
Duplication
Portion of the chromosome has
been doubled
Deletion
Loss of chromosome segment
Inversion
A chromosomal segment is
reversed (rotated 180o)
Translocation
A chromosomal segment is
moved to another
non-homologous chromosome
Mechanisms of Chromosome
Mutation
Breakage and Rejoining
Unequal crossing-over
Duplications
Type of Duplication
Description/Example
Adjacent-Tandem
Duplicated region is immediately
adjacent to the original segment
ABCBCD
Adjacent-Reverse
Duplicated region is adjacent but the
sequence is reversed
ABCCBD
Displaced
Duplication is located on a different
part of the chromosome or on a
different chromosome.
Meiotic Structures
Type of Mutation Meiotic Structure in
heterozygotes
Duplication
Duplication Loop
(Duplicated homolog forms loop, or both
chromosomes form loops )
Consequences of Duplications
• Altered phenotypes as a result of
imbalances in amounts of gene products
Consequences of Duplications
• Duplication can lead to other genetic
changes
– Alleles with new functions
Consequences of Duplications
• Duplication can lead to other genetic
changes
– Higher degrees of duplication
Deletions
Type of Deletion
Description
Terminal
Loss of the end of a chromosome
Interstitial
Loss of internal portion of
chromosome
Intragenic
Deletion within a gene, acts like a
null mutation
Multigenic
Deletion of multiple genes, often
harmful even in heterozygous
condition
Meiotic Structures
Type of Mutation Meiotic Structure in
heterozygotes
Deletion
Deletion Loop
(Normal homolog forms loop)
Consequences of Deletions
1. Chromosome is lost if centromere is
deleted.
2. Chromosomes with deletions do not
revert to the wild type state.
3. Recombination frequencies between
genes flanking the deletion are
reduced.
4. Deletions are lethal in the
homozygous state.
Consequences of Deletions
5. Multiple defects occur in
heterozygotes due to
a.
imbalances in the amounts
of gene products
Solid tumors can accumulate deletions
Cri du chat syndrome results from
a deletion in chromosome 5
Consequences of Deletions
5. Multiple defects occur in heterozygotes
due to
b.
c.
pseudodominance = recessive alleles on the
non-deleted homolog are expressed
haploinsufficiency = certain genes must be
present in two copies for normal function
Wild Type Wing
Notch Wing
Inversions
Type of Inversion
Description
Paracentric
Centromere is outside inversion
Pericentric
Centromere is within inversion
Meiotic Structures
Type of Mutation Meiotic Structure in
heterozygotes
Inversion
Inversion Loop
(Both homologs form loop)
Consequences of Inversions
1. Generally viable since amount of
genetic material is unchanged.
2. Leads to lethal mutation if breakpoint is
within an essential gene.
Consequences of Inversions
3. Position Effect = Gene regulation can be
altered by a difference in chromosome
location
4. Recombination frequency is reduced due to
inhibition of pairing and since crossover in
heterozygotes yields additional structural
mutations.
Crossing Over in a Paracentric
Inversion Heterozygote
Crossing Over in a Pericentric
Inversion Heterozygote
Anaphase II
Crossing Over in Inversion Heterozygotes
Type of Inversion
Crossover products
Paracentric
Dicentric bridge – breakage
causes deletions
Acentric fragment – lost
Pericentric
Chromosomes with both
duplications and deletions
Inversions in Evolutionary History
Translocations
Type of Translocation Description
Reciprocal
Exchange of chromosomal
segments between nonhomologous chromosomes
Non-reciprocal
Segment from one
chromosome joins a nonhomologous chromosome
Robertsonian Translocation
• Long arms of two acrocentric chromosomes are
joined to the same centromere
• Results in one large chromosome and one small
chromosome
• Smaller chromosome may be lost
Inheritance of Down Syndrome from the carrier of
a Robertsonian translocation, joining the long arms
of chromosomes 14 and 21.
Consequences of Translocations
1. Behave as point mutations
if the breakpoint disrupts
an essential gene.
2. Show new linkage
arrangements.
3. Position Effect = Gene
expression is altered as a
result of the new
environment for
translocated material.
Burkitt’s lymphoma is
caused by a position effect.
Consequences of Translocations
Position-effect variegation of eye color in fruit flies due
to translocation of w+ next to heterochromatin
Translocations found in cells of solid tumors
Consequences of Translocations
4. Leads to semisterility, where approximately half
of the gametes carry structural mutations.
Meiotic Structures
Type of Mutation Meiotic Structure in
heterozygotes
Translocation
Cross
(Involves both homologs)
Meiotic Segregation in Translocation Heterozygotes
Meiotic Segregation in Translocation Heterozygotes